An ideal ultrasound imaging system will maintain its imaging resolution at an optimum value throughout the area of interest. One method for accomplishing this is often referred to as beamformation with the complete data set or N.sup.2 reconstruction. With this method, the data acquisition sequence proceeds as follows: transmit with transducer element 1, receive with transducer elements 1 through N; transmit with transducer element 2, receive with transducer elements 1 through N; and so forth.
Since this approach requires N.sup.2 transmit/receive operations, it is clearly not feasible for clinical imaging due to the data acquisition time requirements. However, it does permit a beamformation process in which each individual pixel of the image has its own specific set of beamformation parameters. By this method, one can achieve dynamic focusing on transmit beamformation as well as on receive, so that N.sup.2 reconstruction is often considered the target or point of comparison by which clinically feasible approaches are measured. Thus, a data acquisition method that approaches the N.sup.2 method while minimizing frame rate impact would be desirable.
A conventional ultrasound image is composed of multiple image scan lines. A single scan line (or small localized group of scan lines) is acquired by transmitting focused ultrasound energy at a point in the region of interest, and then receiving the reflected energy over time. The focused transmit energy is referred to as a transmit beam. During the time after transmit, one or more receive beamformers coherently sum the energy received by each channel, with dynamically changing phase rotation or delays, to produce peak sensitivity along the desired scan lines at ranges proportional to the elapsed time. The resulting focused sensitivity pattern is referred to as a receive beam. Scan line resolution is a result of the directivity of the associated transmit and receive beam pair.
Output signals of the beamformer channels are coherently summed to form a respective pixel intensity value for each sample volume in the object region or volume of interest. These pixel intensity values are log-compressed, scan-converted and then displayed as an image of the anatomy being scanned.
The frame rate of a medical ultrasound imaging system is determined by the number of transmit events necessary per frame. In conventional ultrasound imaging systems a transmit event is a focused beam transmitted in a particular direction or at a particular focal position. Frame rate in medical ultrasound imaging is a valuable resource. With increased frame rate, larger regions (as in color flow or three-dimensional imaging) or faster objects (e.g., the heart) can be imaged. In addition, image enhancement methods such as video integration (noise reduction) or compounding (speckle reduction) can also use up frame rate.
In conventional medical ultrasound imaging, a single pulse is transmitted in a particular direction and the reflected echoes are coherently summed to form a single line in the image frame. The amount of time necessary to form that scan line is determined largely by the round-trip transit time of the ultrasonic pulse. Furthermore, many scan lines are present in an image frame to densely sample the anatomical region of interest. Thus, the frame rate in conventional medical ultrasound imaging is determined by the sound propagation speed and the size of the region of interest.
High-frame-rate systems are desirable for present 2D (two-dimensional) imaging and necessary for future real-time 3D (three-dimensional) imaging. The frame rate can be improved by decreasing the number of transmit events per frame. This has been conventionally accomplished with a proportional reduction in the number of transmit elements used in each transmit event, resulting in poor signal-to-noise ratio (SNR).
Conventional ultrasound beamformers use dynamic focusing during reception of echoes. With this method, the beamformation process is optimized for each depth to achieve as good a beamshape (i.e., narrow beamwidth with low sidelobes) as possible. However, in most systems, a single fixed focus is used during transmit beamformation to try to maintain a good combined beamshape. In areas away from the transmit focus, the beamwidth of the resultant beam widens and the sidelobes increase.
In one known ultrasound imaging system, an improvement to the focal properties is achieved by using multiple transmits aimed at different focal locations or zones. The echoes from these focal zones are used to form subimages, which then are "stitched" together in the final image. While this method optimizes beam properties in most areas of the image and hence begins to approximate N.sup.2 performance, this is a major penalty of frame rate, i.e., the speed of sound is sufficiently slow to bring the frame rates down to as low as 5 frames/sec. In typical cases as many as eight transmit focal locations are used, which brings about an 8-fold reduction in frame rate. This penalty is quite severe with lower-frequency probes which are used in clinical situations requiring deep penetration.
A similar limitation associated with data acquisition time occurs even more seriously with color flow mapping, a Doppler-based technique in which typically 4 to 16 transmissions are made in a direction of interest to acquire enough data for clinical utility. One approach that has been implemented to try to overcome this limitation is that of transmitting a wider beam and placing multiple receive beams within the transmit envelope. The resultant beams are not necessarily of good quality; however, given the relatively modest needs of Doppler processing, the method works satisfactorily. The quality of such beams is not sufficient for B-mode imaging.
One attempt to acquire data at a faster rate and with sufficient image quality is disclosed in commonly assigned Thomenius et al. U.S. patent application Ser. No. 09/197,744, filed Nov. 23, 1998. That application discloses a method and apparatus for acquiring data in high-frame-rate high-resolution (i.e., low f-number) ultrasonic imaging. The technique involves transmitting multiple physically separated beams simultaneously and acquiring imaging data for more than one scan line during receive. Spatial apodization is used to influence the transmit beamformation and to form two controlled and focused spatially separate beams with a single firing of the transducer array elements and without use of additional timing electronics. This method is referred to as "dual beam steering by apodization". Dual beam steering by apodization involves transmission of a single time-delayed focused signal that is separated simultaneously into two distinct beams by imposing a cosinusoidal apodization of the transmitted signals from the elements of the transmitting phased array. The method can be extended to provide improved performance for larger scan angles and larger angular separations of the dual beams. Also the concept is extended to multiple (more than two) transmit beams with a single transmit firing. This concept can also be applied to two-dimensional arrays making it possible to work with a two-dimensional set of transmit beams.
In ultrasound multiline acquisition, multiple receive beams are acquired from a single transmit pulse. An artifact in this acquisition mode is the line warping which can occur when the transmit beam is focused in-between receive beams. Due to the changing transmit beamwidth, the effective receive beam location is pulled towards the center in the area of the transmit focus. Thus there is need for a technique capable of solving the line warping problem.